1. Field of the Invention:
The present invention relates to a detector for detecting the ratio of air to fuel, i.e., the air-fuel ratio of an internal combustion engine or the like.
2. Description of the Prior Art:
It is the current practice to detect an oxygen concentration in the exhaust gas discharged from an internal combustion engine of an automobile or the like so that the amounts of air and fuel to be fed to the internal combustion engine may be controlled on the basis of the detected value to thereby reduce the noxious content in the exhaust gas.
A known air/fuel ratio detector (or oxygen sensor) makes use of the principle of the "oxygen concentration cell" and can detect a stoichiometric air/fuel ratio (i.e., A/F=14.6) but not other regions such as a lean range in which the air/fuel ratio is higher than the stoichiometric value, or a rich range in which the air/fuel ratio is lower than this ratio. On the other hand, there has been developed a limit current type oxygen sensor for detecting the oxygen concentration by making use of a known phenomenon, and it has been investigated to detect the air/fuel ratio in the lean range. According to that phenomenon, oxygen ions will permeate from a cathode to an anode of air-permeable thin electrodes which are placed on the two sides of a solid electrolyte cell permeable to oxygen ions, if a voltage is applied between those two electrodes, so that a current will accordingly flow between the two electrodes, but the current will not increase more than a predetermined amount even with an increase of the applied voltage if the amount of permeation of the oxygen ions is restricted. The limit current type oxygen sensor is called a lean sensor because it can detect an air/fuel ratio in the lean range but can hardly detect an air/fuel ratio in the rich range.
However, in the case of the automobile, for example, it is preferable to normally run the engine in the lean range. In an uphill run requiring a higher power output, however, it is preferable to run the automobile in the rich range. It has therefore been desired to provide a detector which can cover an air/fuel ratio from the rich range to the lean range.
In order to satisfy the above-specified desire, the present Applicants have proposed an air/fuel ratio detector as shown in FIG. 13 which is a sectional view showing one example of the air/fuel detector according to the prior art. Reference numerals 1 and 4 appearing in FIG. 13 denote tubular solid electrolyte elements permeable to oxygen ions, which are equipped on their respective inner and outer sides with electrodes 3a and 3b, and 5a and 5b, made of platinum or the like. Moreover, the element 1 has its closed end formed with a gas diffusion hole 2. Numeral 6 denotes a sealing member, numeral 7 a heating element, numeral 8 a tubular heater, numerals 9, 10 and 11 leads and numeral 12 an insulating tube.
In this air/fuel ratio detector, the element 4 is used as an oxygen pump for pumping oxygen from the inside (which is vented to the atmosphere) of the element 4 into a space defined by the elements 1 and 4 so that the concentration of the residual oxygen may be detected by the element 1 acting as the limit current type oxygen sensor after the residual oxygen has reacted with the unburned content in the exhaust gas diffused through the gas diffusion hole 2.
The air/fuel detector thus constructed has output characteristics (i.e., V-I characteristics), as shown in FIG. 14. As is apparent from FIG. 14, the output characteristics, as expected, were obtained at the lean side (i.e., A/F=15 to 17) but are considerably different from the expected curves (as depicted by broken curves) at the rich side (i.e., A/F=12 to 14). This is because the space defined by the elements 1 and 4 is always fed with the oxygen from the atmosphere by the oxygen pump (i.e., the element 4) and is always held in a lean state so that the element 1 acts as a concentration cell to generate an electromotive force between the two electrodes 3a and 3b in case the exhaust gas is rich. As a result, the V-I characteristic curve at the rich side are caused to become different from the expected shapes by the influence of that electromotive force.
More specifically, the aforementioned space is fed with oxygen by the oxygen pump (i.e., the element 4). In the prior art, the electrode placed on the outer side of the element 1 is made of platinum or a material having a catalytic action for purifying the exhaust gas and is in direct contact with the exhaust gas so that the element 1 acts as the concentration cell to generate the electromotive force. Where the concentrations of the oxygen contacting with the two electrodes are very different, the element acts as the concentration cell to generate the electromotive force between its two electrodes. For example, the oxygen concentration at the side of the inner electrode of the element 1 is always held no lower than 10.sup.-1 vol % by the oxygen pump action of the element 4. When the oxygen in the rich state comes into contact with the outer electrode of the element 1, on the contrary, the oxygen concentration drops to 10.sup.-20 to 10.sup.-30 vol %, e.g., 10.sup.-27 vol % by the catalytic action of the electrode so that the element 1 acts as the concentration cell.